OPA858: Power supply decoupling

Hi David,

I would do it as shown in the EVM:

I would make a direct connection between the VS- pin of OPA858 and its pad. Or by other words, I would isolate the pad from the negative power plane.

By the way, I think the 20R resistor in question was meant to be installed in series to the IN+ input (non-inverting input) of OPA858, not in series to the power supply pin.

Kai

Kai,

Thanks for the guidance. I confused the 20 ohm resistors with the following statement in the support response  ...

"2. As being a 5.5GHz OPAmp you need to consequently apply HF design practises. This especially means to use supply voltage filtering by the help of series resistances or ferrite beads. Apply this for each single OPAmp and each single supply voltage. Put the series resistors and/or ferrite beads closest to the decoupling caps which themselves should sit closest to the supply pins of OPAmps. This alone could already do the trick and prevent oscillations.

By the way, I always use PI filters for this purpose. When doing so there's no need for an extra power supply plane."

We will connect the thermal pad directly to the VS- pin. The power pins are filtered with a PI ... C-R-C with R = 20 ohms. Also, we added a series 20 ohm resistor on the Output and the IN+ pin. (Our application is a non-inverting amplifier.)

Thanks again!